1. Field of the Invention
This invention relates to an entirely novel optical recording method and reproducing method and recording apparatus and reproducing apparatus utilizing stimulated photon echo.
More particularly, the invention utilizes a recording medium which allows a Hole Burning memory persistently or transiently.
2. Related Background
An optical recording method which permits writing at random is generally achieved by rotating a disk-like optical recording medium and modulating the intensity of a narrowly diaphragmed light spot on the medium in response to binary-coded information to be recorded, thereby generating binary bits onto a recording layer.
In the above case, because the bits generated on the recording layer are two-dimensional, a narrower light spot must be made to allow higher recording density, and thus optical diffraction limits determine a recording density. Proposals is using much greater dimensions have been made to break through such two-dimensional writing limits. Above all, methods utilizing the wavelength dimension of a writing light have been extensively researched. The methods are generally referred to as a Hole Burning memory. Of these, one whose memory is not transient (information is not lost in relatively short time) but persistent is specifically called Persistent Spectral Hole burning or Photochemical Hole Burning or PHB or PSHB in short.
The record reproducing methods using a recording medium which permits use of the Hole Burning memory includes the following two methods:
(1) Method according to a frequency domain memory
A record reproducing method utilizes the wavelength dimension of a writing light. A narrow banded variable wavelength laser is used as writing and reading lights to write and record a wavelength-controlled hole (a peak where transmissivity increases in the manner of selecting the wavelength by light illumination) in zero photon absorption band inhomogeneously spread on a recording medium.
(2) Method according to a time domain memory
In this method, by employing pulse lights as reading and writing lights, basically a phenomenon called a stimulated photon echo is utilized to record the time correlation of the two pulse lights. At this stage, a uniquely shaped hole is recorded corresponding to the time correlation of the pulses in a wavelength space of the recording medium.
The conventional method of the time domain memory is herein described further in detail and some of its problems are clarified.
As is well known, a light excited state of a substance is expressed by an equation of motion of its density matrix (Liouville equation). For the sake of convenience, the relaxation time of density matrix diagonal element is called T1 time (vertical relaxation time) as distinguished from the relaxation time of density matrix non-diagonal element, which is called T2 time (horizontal relaxation time). A vertical relaxation is considered to mean a process of relaxing the light excited state with an energy release and a horizontal relaxation is regarded as a process to disturb the coherence of the electrical polarization vibration in the Substance brought on by an incidence light.
The photon echo phenomenon is considered to be a type of third dimensional non-linear optical effect. The stimulated photon echo in the phenomenon is described in connection with FIG. 5.
In the case where a substance is assumed to be excited by a proper pulse light in an energy resonance manner, the light of E0 is first incident on a time origin and the light of E1 is then incident on the t1, and a third pulse E2 is incident on t2, a light is then in return reflected from a substance to a (t1+t2). This is a photon echo light. If the Liouville equation is calculated by the disturbance development of rotating wave and weak excitation light approximations, and it is assumed that the inhomogeneous width of the substance is wider than that of a excitation spectrum, the amplitude of an electric field vector P(t) of an echo in a certain space direction in phase matching is then obtained n the following equation. ##EQU1##
For simplification, if E0, E1 and E2 are assumed to be very narrow time widths, the equation can be formulated to the following equation (2). ##EQU2##
In the above case, the intensity an echo light .vertline.P.vertline.2 for t1 and (t2-t1) is attenuated by EQU exp (-t2-t1/T2), EQU [exp (-2(t2-t1)/T1],
respectively,
where T2 is the aforementioned horizontal relaxation time and T1 the vertical relaxation time. PA1 (1) Since a PHB recording medium whose memory time is relatively long (persistent) usually tends to have a short time T2, an ultra short light pulse must be employed for the data light when recording voluminous information. PA1 (2) In the conventional method, the time change of a light intensity must be observed in order to detect and decode the echo light reproduced. PA1 where ##EQU4## where, GW and GR represent respectively the time correlation function of E0 and E1, and E2 and E3.
As is clear from the above equation, for the generation of the stimulated photon echo, an element relaxed at the T1, that is the diagonal element of the density matrix in a state has a significant meaning. When the diagonal element of the density matrix after the E1 pulse is illuminated, the following equation is obtained. ##EQU3## [where, W=1+cos .omega.t1]where the wave vectors of E0 and E1 are almost the same and sufficiently short in their pulse widths. The .OMEGA. is a resonance angle frequency in a two-level system. The above shows that the state distribution within the in homogeneous width is modulated by the amount in relation to the time t1. This is called a population grating. It may be interpreted that a photon echo light (wave) is diffracted by the population grating.
Basically, in the time domain memory using the stimulated photon echo, either the E0 or E1 becomes a recording excitation light or a data light and both of them form a writing light.
In a read process, the E2 becomes a reproducing excitation light, forming a reading light together with the photon echo light generated. For example, reproduction of the data light by the photon echo is shown in FIG. 6, where the E0 is used as a recording excitation light and E1 as a data light. At this stage, the data light is in digital signals and the reproducing excitation light E2 is equal to E0 for convenience, though it is not necessarily equal.
Thus, in the time domain memory, the amount of recording information is governed by the time T2 and the recording time by the time T1.
If a state, other than the two-level system relating to light absorption, involving a long relaxation time generally called a bottleneck state exists, the time T1 is relatively prolonged. When the light excited state can change, as a PHB recording medium does, to a chemically metastable state called a production state, the population grating in a ground state is semipermanently maintained at low temperatures. In this case, even though the (t2 to t1) in FIG. 5 is prolonged infinitely, the data light can be reproduced as the echo light by means of a reproducing excitation light.
In a system where the time T1 is adequately longer than the time T2, the repeated process of writing at t1&lt;T2 will accumulate the modulation element at the density matrix diagonal element. In this case, a sufficiently weak light is capable of generating a relatively intense echo light. This is generally called an accumulated photon echo process.
The conventional method using such basic applications as the above (e.g. Refer to Opt. Commu., 65, 185 (1988), Opt. Lett., 13 536 (1988) and Opt. Lett., 11,724 (1986) has the following problems:
The ultra short light pulse is, however, subject to environmental influence and up to now has some problems regarding generation.
Accordingly, the pulse width of a light pulse regarded as being suitable for practical use has a limitation of about one picosecond, which leads to a comparatively small memory capacity.
A high speed photo detector, such as a streak camera, now has a time resolution of at most one picosecond and also its sensitivity does not improve even though the time resolution of the detector improves. As stated before the intensity of the echo light in the data pulse matrix is attenuated by the exp (-4 t1/T2).
In a persistent large capacity time memory, there are no light detectors with a wide dynamic range which decode the echo light at high data transmission and efficient S/N ratios. Thus, in the conventional method, information in the persistent large capacity time memory cannot be reproduced at high data transmission and efficient S/N ratios.